For treating water used in atomic power plants, steam power plants and pharmaceutical companies as well as waste water therefrom, filters of precoated type which comprise ion-exchange resin powder as the precoating material are used. The methods in which water to be treated is passed through a filter layer made of a precoating material precoated on a support so as to remove impurities contained in the water are collectively called precoat filtration. Recently, so called precoated filters comprising ion-exchange powder precoated on a support element by water pressure are used and water to be treated is passed through the precoated layer.
When the difference in the pressure exerted to each side of the filter reaches a prescribed level, the precoating material is removed from the support element by back wash and is replaced with a new one. In most cases, before the entire ion-exchange capacity is fully consumed, the difference in the pressure reaches the prescribed level, so that the time point of the back wash is determined by the pressure difference. In the precoat filtration in atomic power plants, all of the wasted precoating material collected by the back wash must be stored because it contains radioactive substances. Increase in the volume of this radioactive waste presents a new social problem.
It is thus desired to prolong the duration from the precoating of the precoating material to the back wash (water-treatment lifetime of the precoating material) as long as possible. This cannot be attained by merely prolonging the water-treatment lifetime of the precoating material by preventing an increase in the pressure difference of the precoating material because the prolongation of the lifetime of the precoating material is not meaningful unless the quality of the treated water is better than or comparative to that of the water treated by a conventional precoated filter.
If the quality of the treated water is improved, the exposure of the staff working in atomic power plants to radioactivity is largely reduced.
To satisfy these demands, a method was proposed in which ion-exchange fibers are used as the precoating material (Japanese Laid Open Patent Application (Kokai) No. 55-67384). However, the advantageous effect brought about by the ion-exchange fibers is merely to prevent the generation of cracks in the precoating material, so that the above-described tasks are not at all attained by this precoated filter. Further, the quality of the treated water is not at all improved by this precoated filter.
A precoating material made of a mixture of ion-exchange resin powder and ion-exchange fibers consisting of ion-exchange polymer and a reinforcing polymer is disclosed in Zosuiqijutsu 14, No. 2, 49 (1988). However, in this reference, a specific combination of the ion-exchange fibers and ion-exchange resin powder, which is very important for the formation of the precoated layer is not disclosed. If a mixture of, for example, a cation-exchange resin powder and anion-exchange fibers, or a mixture of an anion-exchange resin powder and cation-exchange fibers is coated, although the volume of the precoated layer is increased so that the prolongation of the lifetime of the precoating material is attained, the quality of the treated water is degraded. On the other hand, if a mixture of a cation-exchange resin powder and cation-exchange fibers, or a mixture of an anion-exchange resin powder and an anion-exchange fibers is precoated, the precoated layer is too dense and so the pressure difference is very high, so that the object cannot be attained.